This invention relates generally to Pseudomorphic Heterojunction Electron Mobility Transistors (PHEMTs) and more particularly to methods for modeling such transistors.
As is known in the art, PHEMTs are used in power amplifiers and low noise amplifiers in a wide range of military and commercial applications. Because of its high efficiency operation, high gain (even near pinch-off) and low noise figure, the PHEMT has replaced the MESFET in radar and communication applications for frequencies ranging from S-Band through mm-wave. The excellent performance of the device, however, is not backed by adequate large signal models essential in MMIC power amplifier design to accurately predict output power, power added efficiency (PAE), harmonic power, third order intermodulation distortion, and adjacent channel power ratio.
Traditionally, the model formulations developed for the MESFET have been adapted to the PHEMT with mixed results, based on the frequency of operation. The error in this approach has been that one of the most critical model parameters; namely, the gate charge has been modeled with simplistic formulations. As a result, the input capacitance nonlinearities have been inadequately modeled, resulting in inaccurate large signal prediction of fundamental quantities such as power and efficiency with even larger discrepancies in harmonic power and 3rd order intermodulation distortion. Recent papers such as: "Analytical Charge Conservative Large Signal Model for MODFETs Validated up to mm-Wave Range" by R. Osorio, M. Berroth, W. Marsetz, L. Verweyen, M. Demmler, H. Massler, M. Neumann, and M. Schlechtweg, published in the IEEE MTT-S Digest, pp.595, 1998; and "Improved Prediction of the Intermodulation Distortion Characteristics of MESFETs and PHEMTs Via a Robust Nonlinear Device Model", by V. I.Cojocaru and T. J. Brazil, published in IEEE MTT-S Digest, pp.749, 1998, have used more complex expressions for the gate charge with excellent results, but the model verification has been limited to small periphery devices, generally less than 400 .mu.m.